Fuel assemblies of nuclear reactors cooled with pressurized water, and more generally fuel assemblies of nuclear reactors cooled with light water, usually comprise a cluster of parallel fuel rods maintained in a structure comprising spacer-grids for transversely maintaining the rods and arranged in the longitudinal direction of the cluster.
These fuel assemblies have a prismatic shape, for example with a square section and planar lateral sides. The spacer-grids are constituted by metal plates assembled at right angles so as to constitute a network of square meshes for receiving the fuel rods.
In addition to the transverse maintenance of the rods, the grids perform other functions such as the guiding of the cooling fluid circulating in contact with the fuel rods of the assembly. The plates constituting the grids are consequently cut out and bent so as to constitute guide fins for the cooling fluid.
The spacer-grid of square shape has planar outer sides constituted by plates assembled at right angles.
The structure of the assembly further comprises guide tubes fixed to each of the spacer-grids and connected at their ends to an upper nozzle and a lower nozzle of the assembly. These nozzles also include planar lateral sides which are disposed, in the same way as the outer lateral sides of the spacer-grids, in planes defining the planar lateral side of the assembly of prismatic shape.
In the course of the maintenance operations required for periodically unloading and reloading the core of the reactor, some spacer-grids may be damaged so that it is no longer possible to reload the fuel assembly in a place of the core of the reactor without carrying out certain repairs.
This is true in particular when the parts of the grid, such as the guide fins, have been bent outwardly and project from the outer planar side of the fuel assembly. It is then necessary to machine the spacer-grid, for example by grinding, so as to eliminate the projecting parts that prevent its loading inside the core of the reactor.
In the case where a spacer-grid has a damaged part in the vicinity of one of its outer sides, for example that located on a corner of the spacer-grid, it may be possible to eliminate this damaged part and to place a mini-grid for maintaining the rods in the place of the damaged grid part. The damaged grid part may be removed by cutting or grinding in the manner described in FR-A-90-16496 filed by FRAMATOME and COGEMA.
The cutting and grinding operations on the spacer-grids of irradiated fuel assemblies must be carried out under water within a pool. As it might be necessary to machine a grid at any level in the height of the assembly, tools must be provided which are capable of being fixed at any level along the structure of the assembly.
Such tools are complex and difficult to use when the assembly is taken up by handling means, such as a lowering device, or is located in a storage bay at the bottom of the pool.